1. Technical Field
The present invention relates to electromagnetic filtering, and more particularly to filtering high-frequency noise signals occurring in an electronic device.
2. Background of the Invention
Recent developments in mobile multimedia products for vehicles incorporate many diverse electronic products, such as computer boards, DVD players, video displays, modems, digital signal processors, into conventional product designs ordinarily having a simple radio. The high frequency and fast rise time of these additional circuits makes meeting electromagnetic compatibility (EMC) standards, especially with respect to radiated emissions, more difficult.
In product designs incorporating one or two devices, such as a radio and/or CD player, the device typically has a conductive housing. Control and display signals are transmitted from the device""s main processor board through its conductive housing to a keyboard and display via a harness cable, flex circuit or a direct board-to-board connector. Any filtering of the signal transmission lines is done on the processor board. With optimized circuits, such as devices with typical clock frequencies of around 4 MHZ, the radiated emissions from the device can meet acceptable EMC levels.
Mobile multimedia devices, by contrast, often employ multiple circuits and modules with clock rates of 100 MHz or more, sub-nanosecond rise times, expanded mode processors and video signals of 25 MHz or more. These circuits possess both differential signals and common mode noise with harmonics extending on the order of GHz. Board-to-board cables connect multiple modules within the device, and the cables conduct noise throughout the device. Also, electromagnetic fields from the IC""s and circuitry on the boards couple to the board-to-board cables, exacerbating noise and radiated emissions.
The higher noise and radiated emissions levels generated by mobile multimedia devices makes it more difficult to keep emissions within EMC standards. Using conventional methods to connect the display and keyboard in mobile multimedia devices will still allow signals and noise to be conducted through the conductive housing and onto the display and/or keyboard. The resulting energy radiation usually exceeds EMC standards, but the processor board in mobile multimedia devices is insufficient for filtering the signals due to the common mode and coupled noise as explained above.
There is a need for a structure that can filter signals emitted from devices, such as mobile multimedia devices, that generate higher noise levels.
Accordingly, the present invention is directed to a filtering device for controlling emissions from an electronic device having a conductive housing, comprising at least one filter component, a substrate supporting said at least one filter component, and a ground plane supporting said substrate, wherein the ground plane is to be connected to the conductive housing such that noise from the electronic device is grounded to the conductive housing via at least one of the ground plane and said at least one filter component.
The invention is also directed to an electronic device comprising a conductive housing, at least one device circuit, and a filter board for controlling emissions from said at least one device circuit, the filter board including at least one filter component, a substrate supporting said at least one filter component, and a ground plane supporting said substrate, wherein the ground plane is connected to the conductive housing such that noise from the device travels through said at least one filter component and is grounded to the conductive housing via the ground plane.
The invention is further directed to a method for filtering signal emissions from an electronic device having at least one device circuit, comprising the acts of placing a filter board having at least one filter component and a ground plane and connecting the ground plane to the conductive housing such that noise from the electronic device is grounded to the conductive housing via at least one of the ground plane and said at least one filter component.